The present invention relates to an implantable cardioverter or defibrillator system wherein a high-energy pulse is delivered to the heart of a patient. The high-energy pulse is chopped at a high frequency so that the pulse comprises a wave packet formed of a plurality of high-frequency cardioverting pulses.
In recent years, substantial progress has been made in the development of cardioverting techniques for effectively cardioverting, including defibrillating, various heart disorders and arrhythmias. Past efforts have resulted in the development of implantable electronic defibrillators which, in response to the detection of an abnormal cardiac rhythm, discharge sufficient energy via electrodes connected to the heart to depolarize and restore it to normal cardiac rhythm.
"Cardioverting" or "cardioversion" as used herein is intended to encompass the correction of a number of arrhythmic heart conditions, both lethal and non-lethal. These arrhythmic heart conditions include atrial tachycardia, atrial flutter, atrial fibrillation, junctional rhythms, ventricular tachycardia, ventricular flutter, ventricular fibrillation, and any other non-pacemaking related arrhythmic conditions which may be corrected by applying electrical shocks to the heart. The term "defibrillation" is included in the term cardioversion as a method of applying electrical shocks to the heart to defibrillate fibrillating atria or fibrillating ventricles.
Prior art implantable cardioverting systems typically include the detection of an arrhythmia condition which in turn initiates the charging of a storage capacitor by a high-voltage inverter circuit (or DC-to-DC converter circuit). See, for example, U.S. Pat. No. 4,164,946, assigned to the present assignee. Following the charging of the storage capacitor, and upon determination that a cardioverting or defibrillating shock is to be issued to the heart of the patient via implantable electrodes, the capacitor is coupled across the implantable electrodes and the capacitor voltage is applied thereto. The capacitor discharge provides a single high-voltage pulse, in the form of an exponentially-decaying waveform, across the electrodes, and through the heart tissue, to depolarize the heart.
The delivered high-energy pulse in such cardioversion systems is a single DC pulse and thus the impedance of the heart is substantially a function of the DC impedance component, or resistance, of the heart. The impedance of the heart in response to a single DC voltage pulse applied thereto is relatively low and uniform and thus the pulse is not optimally dispersed throughout the heart, but rather to a relatively localized area of the heart. As a result, in order to depolarize a greater area of the heart, without altering the electrode geometry, higher energies must be employed, which have inherent limits in implantable devices.